1. Field of the Invention
Aspects of the present invention relate to a transistor, a fabricating method thereof, and a flat panel display device therewith. More particularly, aspects of the present invention relate to a transistor, a fabricating method, and a flat panel display device, wherein an amorphous silicon layer formed on an insulating substrate is crystallized into a polycrystalline silicon layer using an SGS (Super Grain Silicon) crystallization method, whereby the substrate undergoes a first annealing process by absorbing or diffusing an extremely small amount of metal catalysts into a capping layer as the substrate undergoes a first heat treatment in order to control the concentration of the metal catalyst left in the polycrystalline silicon layer, then the substrate undergoes a second annealing process, that is, a remaining crystallization as the substrate undergoes a second heat treatment after the capping layer and the metal catalyst layer are removed.
2. Description of the Related Art
In general, a thin film transistor (TFT) is a semiconductor device wherein a channel region, where a hole or an electron can flow, can be formed by doping a P-type or an N-type dopant on a source region and a drain region, and then applying a predetermined voltage to a gate electrode. The thin film transistor can be classified into a PMOS (P-type metal-oxide semiconductor) transistor and an NMOS (N-type metal-oxide semiconductor) transistor. If the source region and the drain region are doped with the P-type dopant, and the hole flows when a channel region is formed, the transistor is called the PMOS transistor. On the contrary, if the source region and the drain region are doped with the N-type dopant, and the electron flows when a channel region is formed, the transistor is called the NMOS transistor.
The thin film transistor is used widely as a switching transistor or a driving transistor of a variety of flat panel display devices, such as an active matrix liquid crystal display device and an organic light emitting diode display device. In general, in the thin film transistor as described above, amorphous silicon is deposited on a substrate made of glass, quartz, plastic or steel. Then, a semiconductor layer is formed by crystallizing the amorphous silicon after the amorphous silicon is dehydrogenated. Specifically, the semiconductor layer is formed by depositing an amorphous silicon layer on the substrate using a chemical vapor deposition method, and processed to include a source region, a drain region and a channel region (referred to as an active region as a whole).
However, if the amorphous silicon is deposited directly on a substrate by the chemical vapor deposition method and the like, the amorphous silicon layer containing about 12% of hydrogen is formed, which has low electron mobility. In addition, if the amorphous silicon layer of low electron mobility is crystallized into a silicon layer with a crystalline structure having high electron mobility, the silicon layer can be damaged by an explosion of the hydrogen contained therein. Accordingly, a dehydrogenation process is executed in order to prevent the explosion of the hydrogen during the course of the crystallization process.
In general, an amorphous silicon layer is dehydrogenated by heat-treating the amorphous silicon layer for dozens of minutes to a few hours at temperatures over 400° C. in a furnace. Subsequently, a crystallization process to crystallize the dehydrogenated amorphous silicon layer is carried out.
A few methods used to crystallize the amorphous silicon into polycrystalline silicon include solid phase crystallization, excimer laser crystallization, metal induced crystallization, and metal induced lateral crystallization. The solid phase crystallization is a method wherein an amorphous silicon layer is annealed for a few hours to dozens of hours below about 700° C. 700° C. is a deflection temperature of glass which forms a substrate of a display device to which a thin film transistor is applied. An excimer laser crystallization is a method wherein an amorphous silicon layer is heated locally to a high temperature by injecting a light beam from an excimer laser into the amorphous silicon layer so that the amorphous silicon layer is crystallized.
A metal induced crystallization is a method wherein a metal such as nickel, palladium, gold, aluminum, and so on, is brought into contact with an amorphous silicon layer or injected into the amorphous silicon layer. Consequently, a phase transition is induced during which the amorphous silicon changes into polycrystalline silicon. A metal induced lateral crystallization is a method wherein silicide generated from the reaction of a metal and silicon continuously propagates laterally. Consequently, metal induced lateral crystallization induces the crystallization of an amorphous silicon layer in sequence.
However, the solid phase crystallization has disadvantages in that the process time is too long, and the substrate can be deformed easily due to the high temperature heat treatment occurring over a long time. The excimer laser crystallization has disadvantages in that the process requires an expensive laser device, and the interfacial property between a semiconductor layer and a gate insulating layer is poor because extrusions are generated on a polycrystallized surface. The metal induced crystallization or the metal induced lateral crystallization have disadvantages in that a leak (or leakage) current of a semiconductor layer of a thin film transistor increases because metal catalysts are left in the polycrystalline silicon layer. Consequently, the characteristics of various flat panel display devices using the silicon layer as a switching transistor or a driving transistor of the above processes are poor.